Heterojunction field-effect transistors (HFETs) are commonly used for applications requiring low noise and high power. These transistors typically contain a channel layer surrounded by a barrier layer and a buffer layer. Generally, there are two types of HFETs: single-heterojunction field-effect transistors (SHFETs) and double-heterojunction field-effect transistors (DHFETs). In SHFETs, the buffer layer and channel layer are comprised of the same material, and the barrier layer is comprised of a different material. The channel/barrier interface is the single heterojunction in this structure. In DHFETs the buffer layer and barrier layer are comprised of different materials than the channel layer. Thus, the buffer/channel and channel/barrier interfaces are both heterojunctions. The key feature of a HFET is that the channel/barrier heterojunction induces a highly conductive, two-dimensional electron gas (2DEG) in the channel near the interface.
Shown in FIG. 1 is a typical structure for a HFET. The HFET comprises a substrate 1, and a nucleation layer 3. The substrate 1 typically comprises GaN, AlGaN, SiC, diamond, sapphire, AlN, BN, or LiGaO2. A buffer layer 5 is located on the nucleation layer 3 followed by a channel layer 7 and then a barrier layer 9. The nucleation layer 3 provides a crystallographic transition between the substrate 1 and the buffer layer 5, which may have different crystal structures. The channel layer 7 allows electrons in the channel to flow between the ohmic metal contacts 13, which typically act as the source and drain of the HFET.
The barrier layer 9 induces a highly conductive, two-dimensional electron gas (2DEG) in the channel layer 7 near the interface with the barrier layer 9 and also acts as an insulator between the gate 15 and the channel layer 7. When electrons “spill” from the channel layer 7 into the buffer layer 5, the performance of the transistor is reduced; thus, confinement of electrons in the channel layer 7 is highly desirable. The barrier layer 9 is located on the channel layer 7. A cap layer 11 is also provided on a portion of the barrier layer 9. The cap layer 11 helps prevent oxide and other impurities from damaging the barrier layer 9 during processing. Ohmic metal contacts 13 are also provided. The ohmic contacts 13 are annealed at a high temperature such that they diffuse into the cap layer 11 and barrier layer 9, where they contact the channel layer 7.
The following will describe some typical HFETs making reference to the above description and the HFET structure shown in FIG. 1. GaN-based single-heterojunction field-effect transistors (SHFET) are commonly used in the design of GaN HFETs. In a GaN-based SHFET the nucleation layer 3 comprises AlN or AlGaN. The buffer layer 5 comprises GaN, the channel layer 7 comprises GaN, and the barrier layer 9 comprises AlGaN. FIG. 2 is a band-edge diagram depicting the conduction band of a GaN-based SHFET where the buffer layer 5 comprises GaN, the channel layer 7 comprises GaN and the barrier layer 9 comprises Al0.28Ga0.72N. Because the bandgap of AlGaN is larger than that of GaN, there is a band-edge discontinuity at the interface between the barrier layer 9 and channel layer 7. The nature of this discontinuity is such that a potential energy well for electrons is formed in the channel layer 7 near the barrier layer 9. Electrons are confined to the channel layer 7 and a 2DEG is formed. It is important to note that because AlGaN and GaN have different lattice parameters, the interface between these materials is strained. This strain results in positive polarization charges at the channel layer 7 and barrier layer 9 interface. These charges intensify the sharp band-edge discontinuity at the interface between the barrier layer 9 and channel layer 7, further confining electrons to the 2DEG. However, the interface between the channel layer 7 and buffer layer 5 is not a heterointerface; thus there are no differences in bandgap or positive polarization charges and the conduction band is continuous. As a result, it is easy for so-called “hot electrons” to spill into the buffer layer 5 from the channel layer 7. “Hot electrons” are electrons that have sufficient energy to escape the attractive pull of the potential energy well at the interface between the channel layer 7 and barrier layer 9. They are typically present in high-electric field regions of the channel layer 7. The hot electron effect occurs because electron energy-relaxation time is typically significantly longer than their momentum relaxation time. These electrons have sufficient energy to move into another region, such as the buffer layer 5, ultimately degrading the performance of the SHFET.
The confinement of the 2DEG in the channel layer 7 can be improved by using a double-heterojunction structure in which the buffer layer 5 comprises a material having a wider bandgap than that of the channel layer 7. For example, in InP-based HFETs (so called because the device layers are grown on InP substrates), double-heterojunction field effect transistors (DHFETs) have been utilized. DHFETs are also discussed in U.S. Pat. No. 4,827,320 and in Loi. D. Nguyen et al., IEEE Transaction on Electron Devices, vol. 39, pp 2007–2014 (1992). A band-edge diagram of an InP-based DHFET where the buffer layer 5 comprises Al0.48In0.52As, the channel layer 7 comprises In0.53Ga0.47As and the barrier layer 9 comprises Al0.48In0.52As is shown in FIG. 3. As can be seen, the sharp band-edge discontinuities which exist at the interface between the barrier layer 9 and channel layer 7, as well as at the interface between the channel layer 7 and buffer layer 5, help confine the 2DEG to the channel layer 7. Similar results have also been attainable using other wide-gap materials such as GaAs and narrower-gap materials such as InAs. However, InP-, GaAs- and InAs-based materials do not provide the advantages of using GaN-based materials. For example, GaN-based materials have much larger bandgaps than InP-, GaAs-, or InAs-based transistors which allow a higher voltage to be applied to the transistor before entering breakdown.
Designs for DHFETs implemented in GaN-based materials have mimicked designs for DHFETs implemented in InP-, GaAs-, and InAs-based materials. One attempt at demonstrating such an analogous device is discussed in N. Maeda et al., physica status solidi (b) pp. 727–731 (1999). Shown in FIG. 4 is a band-edge diagram of a GaN-based DHFET. The buffer layer 5 comprises Al0.095Ga0.905N, the channel layer 7 comprises GaN and the channel layer 9 comprises Al0.28Ga0.72N. In general though, the buffer layer 5 comprises AlxGa1−xN, the channel layer 7 comprises GaN, and the barrier layer 9 comprises AlGaN, where x is typically in the range of 15%<x<50%. Such a structure is discussed in U.S. Pat. 5,929,467. This Al concentration yields an AlGaN alloy with a bandgap much larger than that of the GaN of the channel layer 7 (just as the bandgap of the Al0.48In0.52As buffer layer is much larger than that of the In0.53Ga0.47As channel layer in InP-based DHFETs). However, such attempts to mimic the 2DEG confinement in GaN, in a manner similar InP-, GaAs- and InAs-DHFETs have proven unsuccessful.
These GaN DHFETs contain large polarization charges at the interface between the buffer layer 5 and channel layer 7. These charges result in exceptionally large electric fields at that interface which cause the valence band edge on the channel layer 7 side to rise above the Fermi level at the interface, as shown in FIG. 4. As a result, a two-dimensional hole gas (2DHG) forms at the interface of the channel layer 7 and buffer layer 5. The 2DHG increases the capacitance of the DHFET, which reduces the performance of the transistor. Furthermore, the 2DHG is poorly controlled by the voltage at the gate 15, and because hole mobility is significantly lower than electron mobility, the frequency response of the DHFET is significantly limited.
As a result, there is a need for a HFET that provides the bandgap characteristics of GaN-based HFETs, confines the 2DEG to the channel layer, and reduces the 2DHG.